The general aim of this project is to study and understand the variability and modulation of neurotransmitter release by single central interneurons onto their different targets in local circuits. We will continue our quantal and statistical analysis of neurotransmission between spiking and nonspiking interneurons, and characterize the variability of quantal contents and release probabilities between the synapses made by single interneurons onto several targets. These studies will be done in the insect central nervous system in vivo, and then extended to those same neurons in culture. We will study the biophysical mechanisms underlying this variability. The importance of cytosolic Ca2+ concentrations, their fluctuations between release sites, the spatial distribution of Na+, K+ and Ca2+ voltage-gated channels on the dendritic arbor of the presynaptic neurons will be studied using electrophysiological, immunocytochemical and imaging techniques. Numerical simulation experiments will be conducted to assess the importance of parameters which cannot be measured yet with the available experimental techniques. We will also examine the role of neuromodulators in modifying the release properties of local interneurons. The effects of amines and peptides known to be present in those circuits will be determined, and some of their molecular mechanisms of action will be studied using the heterologous expression techniques applied in H. Lester's laboratory.